Damage to aquatic ecosystems caused by lake acidification was one of the best-documented effects of acid rain, and had been investigated in the U.S. and Canada since the early 1970s. In 1987, the U.S. National Acid Precipitation Assessment Program (NAPAP) concluded that only “a small fraction” (NAPAP 1987: I-8) of lakes in North America have been acidified. The Canadian acid rain program RMCC attacked this report to be “misleading, flawed, and incomplete” and asserted that not “some”, but “numerous” lakes have been acidified (RMCC 1987: 5). Why did scientists in the US and Canada come to different conclusions about the extent of lake acidification in North America? I discuss several reasons for this dispute, including disagreements about the appropriate way of measuring lake acidity (pH value versus acid neutralizing capacity, ANC), and about the appropriate reference value for pH or ANC. I show that the disagreement about reference values raised complex questions of what biological damages are observed at certain pH and ANC levels. My talk points out that U.S. and Canadian scientists did not only disagree about at which pH or ANC values noticeable or severe damages occur, but also about what a “noticeable” or “severe” damage is. I argue that the debate over lake acidification involved deep disagreements about values that were hardly made explicit, but that substantially influenced the creation of facts about the extent of lake acidification.